This invention relates to diffusion coatings formed on superalloy substrates and specifically to a novel method of renewing the diffusion coatings formed on superalloy substrates.
The current coatings used on superalloy substrates such as airfoils exposed to the hot gases of combustion in gas turbine engines for both environmental protection and as bond coats in thermal barrier coating (TBC) systems include aluminides of nickel and platinum. These coatings are applied over superalloy substrate materials, typically nickel-base superalloys, to provide protection against oxidation and corrosion attack. These coatings are formed on the substrate in a number of different ways. For example, a nickel aluminide, NiAl, typically is grown as an outer coat on a nickel base superalloy by exposing the substrate to an aluminum rich environment at elevated temperatures. The aluminum from the outer layer diffuses into the substrate and combines with the nickel diffusing outward from the substrate to form an outer coating of NiAl. Because the formation of the coating is the result of a diffusion process, it will be recognized that there are chemical gradients of Al and Ni, as well as other elements. However, Al will have a high relative concentration at the outer surface of the article which will thermodynamically drive its diffusion into the substrate creating a diffusion zone extending into the original substrate, and this Al concentration will gradually decrease with increasing distance into the substrate. Conversely, Ni will have a higher concentration within the substrate and will diffuse through the thin layer of aluminum to form a nickel aluminide. The concentration of Ni in the diffusion zone will vary as it diffuses outward to form the NiAl. At a level below the original surface, the initial Ni composition of the substrate is maintained, but the Ni concentration in the diffusion zone will be less and will vary as a function of distance into the diffusion zone. The result is that although NiAl forms at the outer surface of the article, a gradient of varying composition of Ni and Al forms between the outer surface and the original substrate composition. The concentration gradients of Ni and other elements that diffuse outwardly from the substrate and the deposited aluminum, Al, create a diffusion zone between the outer surface of the article and that portion of the substrate having its original composition. Of course, exposure of the coated substrate to an oxidizing atmosphere typically results in the formation of an alumina layer over the nickel aluminide.
In some coating systems, a platinum aluminide (PtAl) coating is formed by electroplating a thin layer of platinum over the nickel-base substrate to a predetermined thickness. Then, exposure of the platinum to an aluminum-rich environment at elevated temperatures causes the growth of an outer layer of PtAl as the aluminum diffuses into and reacts with the platinum. At the same time, Ni diffuses outward from the substrate changing the composition of the substrate, while aluminum moves inward through the platinum into this diffusion zone of the substrate. Thus, complex structures of (Pt,Ni) Al are formed by exposing a substrate electroplated with a thin layer of Pt to an atmosphere rich in aluminum at elevated temperatures. As the aluminum diffuses inward toward the substrate and Ni diffuses in the opposite direction through the Pt creating the diffusion zone, PtAlx phases precipitate out of solution so that the resulting Ptxe2x80x94NiAl intermetallic also contains precipitates of PtAlx intermetallic, where x is 2 or 3. As with the nickel aluminide coating, a gradient of aluminum occurs from the aluminum rich outer surface inward toward the substrate surface, and a gradient of Ni and other elements occurs as these elements diffuse outward from the substrate into the aluminum rich additive layer. Here, as in the prior example, an aluminum rich outer layer is formed at the outer surface, which may include both platinum aluminides and nickel aluminides, while a diffusion layer below the outer layer is created. As with the nickel aluminide coating, exposure of the coated substrate to an oxidizing atmosphere typically results in the formation of an outer layer of alumina.
These aluminides are also used as bond coats in thermal barrier systems, being intermediate between the substrate and an additional applied thermally resistant ceramic coating, such as yttria-stabilized zirconia (YSZ) which is applied over the aluminide. However, the process for forming these diffusion aluminides is essentially the same, that is to say, the substrate is exposed to aluminum, usually by a pack process or a CVD process at elevated temperatures, and the resulting aluminide formed as a result of diffusion.
Over time in the hot oxidizing environment of a gas turbine engine, the coatings, whether applied as an environmental coating or as a bond coat in a thermal barrier system eventually degrade as a result of one or a combination of ongoing processes which include erosion due to the impingement of hot gases on the airfoils, corrosion due to reaction of contaminants in the products of combustion with the metallic surfaces of the airfoil, and oxidation. Products of combustion frequently build up on these outer surfaces. In addition to degradation as a consequence of exposure to the hot engine environment, airfoils may be damaged in service due to a variety of factors, and require repair after removal of damaged regions by well-known processes such as welding, cladding or the PACH process. In order to repair an airfoil after service, it is necessary to remove not only the products of combustion, the corrosion products and oxidation products resulting from routine exposure to the engine environment, but also previously applied coatings, if they haven""t already been removed as a result of service.
Prior art processes for repair of coated blades chemically strip any remaining coating from the turbine blades. One of these repair methods, as set forth in U.S. Pat. No. 4,746,369 involves acid stripping. Because the coatings are grown into the substrate by a diffusion process, acid stripping attacks the diffusion zone, which includes original substrate material, as well as the nickel aluminide and any outer layer of alumina. Of course, this acid stripping procedure is further complicated because the coatings are selected due to their ability to resist chemical attacks from corrosion processes and protect the substrate airfoil. Yet, existing methods of stripping the coatings are controlled chemical attacks upon the airfoil. Unless exceptional care is maintained, the chemical solutions used to remove the coating will vigorously attack the regions underlying the protective coating. So removal of the coating will affect the outer coating layer and the diffusion layer, and may involve a direct attack on the substrate or a portion of the substrate. Because the parts are thin, a repair process that removes at least a portion of the initial substrate that was incorporated into the diffusion zone limits the number of times that the airfoils can be reused since minimum allowable wall thicknesses cannot be violated.
Other methods such as disclosed in U.S. Pat. No. 4,425,185 have as their object removal of coatings such as nickel aluminides from Hastelloy-X substrates without adversely affecting the substrate. This method may minimize the impact on a Hastelloy-X substrate, which has a low Ni content in comparison to a Ni-base superalloy, but it still removes any diffusion zone formed between the nickel aluminide and the substrate. Furthermore, while this may be an effective method for an alloy such as Hastelloy X containing only about 50% Ni, it is not effective for a Ni-base superalloy which can include Ni in excess of 80%.
Another method for removing coatings is set forth in U.S. Pat. No. 5,851,409 to Schaeffer et al. and assigned to the assignee of the present invention. This method involves mechanically impacting the environmental coating at a temperature below the ductile-to-brittle transition temperature of the diffusion zone such as by shot peening. This mechanical action forms cracks in the coating that facilitates penetration of the stripping solution into the coating into the vicinity of the interface between the substrate and the diffusion zone and speeds the removal process. The difficulty with this method is that since there is significant, if not total penetration, of the diffusion zone, including removal of at least a portion of the diffusion zone that was incorporated from the initial substrate, the article is undesirably thinned as a result of the procedure.
What is needed is a method of removing the outermost layer of a nickel aluminide coating applied to a nickel-base superalloy substrate without affecting, or only minimally affecting, the diffusion layer substantially formed from the superalloy substrate, the diffusion layer being located below the outermost layer of nickel aluminide coating. Associated with the removal of the outermost layer of the coating is a method of restoring or renewing the nickel aluminide coating after repairs of the superalloy substrate has been repaired.
The present invention is applicable to nickel-based and nickel-containing superalloy components that operate at elevated temperatures and include aluminide coatings for environmental protection from a harsh environment or an elevated temperature atmosphere such as is found in the oxidative, corrosive exhaust of a jet engine. Typical nickel-base superalloy and cobalt-base superalloy components exposed to such environmental conditions include airfoils in the form of vanes, nozzles and blades, shrouds, combustion liners and augmentor hardware.
The present invention extends the life of this expensive engine hardware by removing the outer layer of the diffusion aluminide coating so that repair of the hardware can be accomplished with little or no removal of the underlying diffusion layer of the diffusion aluminide coating. The protective diffusion aluminide coatings are formed on superalloy components by exposing the component substrates to an aluminum species by using any one of a number of well-known processes. A platinum layer may be electrodeposited onto the superalloy substrate prior to exposure of the component substrate to the aluminum species. The resulting protective coating is formed as a result of diffusion of aluminum into the underlying material, which is either a nickel-base superalloy substrate, a cobalt-base superalloy substrate or a platinum-plated superalloy substrate. The coating has at least two distinct portions overlying the unmodified superalloy substrate. The first portion is an outer portion comprised of a layer that is substantially an aluminide. This aluminide layer is formed as the major elemental components of the substrate, Ni and/or Co, diffuse outward from the substrate and combine with the aluminum in the additive layer. If platinum is present, the aluminide may form PtAl, NiAl, CoAl and combinations thereof, depending upon the chemical composition of the substrate. Because these aluminides are ordered intermetallics formed by diffusion, initially there will be a gradient of Al and Pt, NiAl and/or NiAl and/or CoAl across the outer portion. The second portion is a diffusion layer that has a chemical composition resulting from the high temperature diffusion of elements from the additive aluminum and the substrate, yet different from them. This diffusion layer is intermediate between the unaffected substrate and the outer additive layer and incorporates a portion of the initial substrate. The composition of this layer will vary due to the various elements comprising it. If the substrate includes an electroplated Pt layer, there will be an optional Pt-rich layer between the substrate and the outer additive layer. Upon exposure to an oxidizing atmosphere, any excess Al in the outer additive portion typically will combine with oxygen to form an alumina layer.
In accordance with the present invention, any products of combustion that may have been deposited on the outer layer of the superalloy component are first cleaned off by conventional methods. These methods include light mechanical buffing or cleaning utilizing suitable chemical solvents. The superalloy article then is contacted with a preselected chemical stripping solution for a preselected time. The article is permitted to remain in the solution only for a period of time sufficient to remove at least a portion of the additive outer layer from the substrate. The superalloy substrate is then withdrawn from contact with the chemical stripping solution with at least a portion of the additive outer layer having been removed. The stripping solution on the superalloy substrate is neutralized so that erosion of the remaining coating will not continue.
At the conclusion of the stripping operation, the nickel-based superalloy substrate may be repaired in accordance with established procedures if needed, and then recoated. Repairs can include welding, cladding, PACH, brazing and other established procedures. The article can then be coated in accordance with established methods for applying coatings.
An advantage of the present invention is that only the outer additive layer of the aluminide coating is affected by removal from the article under repair. The diffusion layer underlying the outer additive layer is substantially unaffected.
Another advantage of the present invention is the protective outer layer can be restored by a conventional VPA process that adds a layer of aluminum, allowing the aluminide layer to be restored within the additive layer by diffusion of Ni, Co and combinations thereof from the remaining underlying material so that there is no weakening or thinning of the part due to the stripping process. Alternatively, a thin layer of Pt can be electroplated and complex (Ni, Pt)Al and/or (Co, Pt) Al may be formed.
Still another advantage of the present invention is that it extends the life of the nickel based superalloy article by enabling it to undergo an increased number of repair cycles. The article can be stripped, repaired and recoated without any loss of component thickness, and the repaired article has a restored protective coating that is as effective as the original coating.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.